1. Field of the Invention
This invention relates to an optical coupler, and more particularly to an optical coupler having a grating in an optical waveguide.
2. Description of the Prior Art
With the development of optical integrated circuits, an optical coupler of high accuracy and good efficiency has been keenly desired. However, very fine processing techniques have been required for optical couplers of desired accuracy, and thus there have been problems such as a decrease in yield and a consequent increase in cost.
A description now will be provided of the conventional grating-type optical coupler.
FIG. 3 is a cross-sectional view of the conventional, flat grating-type optical coupler, where numeral 2 is a substrate, which is, for example, a transparent substrate of glass, plastic, or lithium niobate (LiNbO.sub.3), or an opaque substrate of silicon or ceramics. An optical guide 1 is provided on the surface of substrate 2 and has a higher refractive index than that of substrate 2. For example, in the case of a substrate of lithium niobate, the optical waveguide 1 having a film thickness of about 0.5 to about 5 .mu.m can be formed by thermal diffusion of titanium (Ti) on the substrate surface, or in the case of a substrate of glass having a refractive index of 1.5, the optical waveguide 1 having a film thickness of about 0.5 to about 5 .mu.m can be formed by the sputtering of glass having a refractive index of 1.55 (wavelength: 6328 .ANG.), for example, glass C-7059 (trademark of Corning Glass, U.S.A.) on the substrate surface.
The numeral 6 represents gratings usually at a pitch of 0.3 to 1 .mu.m in a depth of 0.1 to 0.2 .mu.m. When an incident light enters 3 an optical coupler of said structure, light 4 propagates through the waveguide by total reflection, and outgoing light 5 as diffracted by the gratings 6 can be obtained.
In that case, it is generally difficult to prepare gratings, and the preparation of a flat type grating is more difficult than that of a relief type. To effectively attain the function of a gratings 6, gratings of complete rectangular form are preferable, but it is generally not realizable to obtain such gratings of complete rectangular form.